An LLC resonant circuit is a common resonant circuit (L is a symbol of an inductor, C is a symbol of a capacitor, LLC represents a resonant circuit formed by two inductors and one capacitor, and the LLC resonant circuit may also be referred to as a series-parallel resonant circuit (SPRC, Series-Parallel Resonant Circuit)). As shown in FIG. 1, main units of a typical LLC resonant circuit include a resonant inductor Lr and a transformer Tr, and the LLC resonant circuit further includes switching devices Q1 and Q2, an excitation inductor Lm of the transformer, resonant capacitors Cr1 and Cr2, a filter capacitor C, and rectifiers D1 and D2. The resonant circuit is connected to a direct current (DC) power supply, energy of the power supply is transferred to a secondary side from a primary side of the transformer in the circuit and is filtered by the filter capacitor C, and filtered energy is converted into an alternating current, and the alternating current is provided for a load R.
During an actual application, a ripple current flowing through a filter capacitor is prone to exceed a standard. Therefore, a three-phase parallel LLC power supply conversion circuit may be used to resolve a ripple current problem. The three-phase parallel LLC power supply conversion circuit is formed in parallel by three identical LLC resonant circuits with a current phase difference of 120 degrees. As shown in FIG. 2, key units of the three-phase parallel LLC power supply conversion circuit include six magnetic elements, namely three resonant inductors LR1, LR2, and LR3 and three transformers with a turn ratio of N1:N2:N2. The three LLC resonant circuits are for parallel output, a phase difference between each two circuits is 120 degrees, a difference between output load currents is 120 degrees, and a phase difference of currents that are output from a secondary side and are rectified is 360 degrees. Therefore, theoretically, output ripple currents may offset one another, that is, a ripple of equivalent currents is 0.
However, the inventor finds that, in a process of implementing the present invention, during implementation, especially in a high-power application scenario, of a three-phase parallel circuit, for example, the foregoing three-phase parallel LLC power supply conversion circuit, because parameters of components in all circuits may not be completely the same in practice, a phenomenon that an operating parameter drifts may exist, and a situation (for example, in FIG. 3, there is a difference among peak values of currents in all branches) in which a current is not equalized in each branch of the three-phase parallel circuit is caused; in this case, output currents of the circuit cannot be completely offset, and a device may even be burnt out in a serious case.